I had the same problem, I have a PCI soundcard but Vector would set the mic of my usb webcam or (when the webcam was unplugged) my onboard sound chip as default sound device, i don't know if this will work for you but worked well for me:
I edited the /etc/modprobe.d/alsa.conf file (i don't remember well right now but i think the file didn't exist so if you don't find the file, just create it)
and added the following line (by the way it is the only line in my file):
options snd slots=snd-emu10k1
my soundcard is a Creative SB Audigy 2, so you replace the emu10k1 part with the name of the driver of your sound card:
basically you can set the order of preference for your sound devices like this
options snd slots=snd-device0,snd-device1,snd-device2...snd-deviceN
but it is no necessary, you don't need to write down all of your devices, just the one you want as default.